The present invention is directed to semiconductor devices and, more particularly, to Schottky diode structures and assemblies.
Schottky diodes are desired for applications where energy losses while switching from forward bias to reverse bias and back can significantly impact the efficiency of a system and where high current conduction is desired under forward bias and little or no conduction is desired under reverse bias, such as when used as an output rectifier in a switching power supply. The Schottky diodes have lower turn-on voltages because of the lower barrier height of the rectifying metal-to-semiconductor junction and have faster switching speeds because they are primarily majority carrier devices.
Presently, most Schottky diodes are silicon-based, vertical conduction devices. In such devices, a Schottky metallic contact is formed on one surface of a silicon body, an ohmic metallic contact is formed on an opposite surface of the silicon body, and when the device is forward biased, current flows vertically across the silicon body from the Schottky metallic contact to the ohmic metallic contact. Silicon-based Schottky diodes have the disadvantage, however, that silicon has a low carrier mobility and a relatively narrow band gap. Further, for higher voltage applications, a thick, low doped base must be used which leads to higher series resistance, a greater forward voltage drop, and increased heat dissipation, thereby making silicon-based Schottky diodes unsuitable for such applications. Moreover, at higher temperatures, the reverse leakage current increases dramatically and negates the rectification properties of the device.
To avoid the problems inherent in silicon-based devices, Schottky diodes made of materials having a higher electron mobility, wider band gap and higher breakdown voltage are desired. Such materials include diamond, silicon carbide, nitride-based semiconductors and other composite materials. These materials, however, are typically formed atop an insulating substrate, and therefore Schottky diodes formed using such materials require a lateral conduction path rather than a vertical conduction path. Devices having a lateral conduction path, however, are prone to high on-resistances when the device is forward-biased because the forward current must travel over a relatively long conduction path determined by the horizontal dimensions of the device. The current must also travel along relatively thin layers of materials having a small cross-sectional area in the direction transverse to the direction of current flow. Moreover, because the forward current flows laterally from the Schottky metallic contact to the ohmic metallic contact, current flow away from the Schottky metallic contact is often non-uniformly distributed so that the current density is crowded along the edge of the contact. The electrically insulating substrate of the lateral conducting Schottky diode is also typically a poorer thermal conductor and thus dissipates heat less efficiently than vertical devices. The poorer heat dissipation increases the complexity of the device packaging because alternative ways of heat removal must be employed. Additionally, the device packaging is further complicated by the presence of both the Schottky metallic contact and the ohmic metallic contact on the same side of the laterally conducting device which requires more complex interconnections than those of vertically conducting devices in which the contacts are on opposite sides.
It is therefore desirable to provide a laterally conducting Schottky diode structure and assembly that provides the advantages of higher electron mobility and wider band gap materials but which are less prone to higher on-resistance, non-uniform current distribution, and poorer thermal conduction and which does not require complex packaging.